1. Field of the Invention
This invention relates to silver recovery from spent photographic solutions, and in particular to an automatic apparatus or system which is particularly useful for recovering silver from spent photographic, x-ray or copying solutions used in chemical printing, developing and copying processes.
2. Description of the Prior Art
In photographic, x-ray and motion picture-type films, a sensitive emulsion is employed to record light or x-ray impressions on the film. The chief constituent of this sensitive emulsion is silver bromide but silver chloride is also used and sometimes a combination of these is employed. When light or x-rays strike the granules of silver bromide or other light-sensitive silver salt in a photographic emulsion, a few atoms of metallic silver are formed in each of the granules. The intensity of the impinging radiation determines the number of metallic silver atoms formed. Granules of the silver bromide containing metallic silver are thereby sensitized, and can be reduced to metallic silver by an organic reducing agent or developer. The developer reduces the exposed granules more easily than the unexposed granules, and by chemically controlling the developer action the non-sensitized regions of the film remain unchanged. The film is then passed through a second chemical solution known as a fixative (commonly called hypo) where a second and very unique action occurs. The unsensitized areas, still containing silver bromide and/or chloride, are washed of the silver salts, which pass into the hypo. When the hypo becomes diminished in fixing power and has to be replaced it contains, in dissolved form, the silver removed from the film. Similar reactions occur in making prints from negatives. In photographic studios, motion picture laboratories, industrial x-ray centers and hospitals, the silver in spent hypo solutions is often wasted by being passed out of the processor directly to the drain, but may sometimes be subjected to recovery.
To date, the most popular system of silver recovery in institutions such as hospitals and photographic arts companies has been the canister method. The system is ineffective for a number of reasons. The canister is usually nothing more than a bucket, ranging in capacity from 5 to 50 gallons, containing iron wool with a number of passages through it, depending upon the manufacturer's design. As the silver-laden solution passes through the iron wool, the iron ions replace the silver ions in solution and metallic silver in the form of sludge appears within the iron mesh. The silver is present with a number of other chemicals and chemical compounds, e.g., iron and iron sulfide, and a multi-stage refining operation is required before any acceptable level of silver purity is attained. The cost of the system is relatively small, varying, depending on size, between $16.00 and $200.00, but it can never be re-used. Additionally, the canister itself has a limited life, because only a certain number of gallons of solution can pass through it before all of its silver removal power is gone. The canister also decreases in efficiency with age, due to internal oxidation and a number of physical reactions undergone as the solution deposits silver in the iron mesh. Typical efficiencies average about 30% to 70%, based on the recoverable silver passed into the canister in solution. A typical life expectancy for such a unit in a hospital or comparable operation is about one month or long enough to treat about 200 gallons of solution.
A second system for removing silver from solution is called "zincing." The system consists of salting or doping large quantities of spent hypo solution with zinc flakes. The solution is then drained off, leaving large quantities of precipitated silver sludge, which must be refined. The operation of this system requires the storing of solutions, which are subsequently usually sold to a zincing operation at a fraction of the solutions'silver worths. In addition, the system is laborious and space-consuming, requiring large users of hypo solution to maintain considerable storage facilities, usually in areas where space is critically needed. The system is a source of only marginal revenue to the hospital in most cases, considering the worth of silver contained in such solutions.
A third system or method for removing silver from hypo solutions is electrolysis or electroplating. This system has the capability of being the most efficient of all processes, due to the fact that the silver is reclaimed in nearly pure form (92% to 99%). However electroplating is also disadvantageous in several ways. It involves a large capital outlay and only when the apparatus and process are automatic and comparatively maintenance-free is it of demonstrable superiority. The prior art machinery normally marketed does not contain simple and reliable automated control apparatuses and usually requres considerable attention. If a machine is left unattended, it will continue at a given current level regardless of fluid flow or solution density, and will decrease in efficiency by losing silver, or begin to cause sulfide formation, noxious gas emission and diminution in the purity of the silver already plated. Operations at voltages and current densities higher than needed place undue strains on parts of the apparatus and the resulting gases emitted can cause considerable discomfort in the area in which the machinery is located. The hydrogen sulfide which can be produced is of a very offensive rotten egg odor which can be a cause for rejection of the apparatus or system for use in quarters also occupied by personnel, as in hospitals and photographic laboratories.
Machines have been marketed with a number of control mechanisms, including concentration sensors, ion migration sensing controls, feed controls, voltage and current regulators and timers, in an effort to have the plating operation conducted most efficiently without production of undesirable byproducts. However, such machines and systems often require adjustment of feed rates or other changes in the feed, with secondary adjustments often being needed so that the machines are not truly automatic, as is the present machine. The prior art machines are often unreliable and are considerably more expensive due to additional equipment included in them. Prior art machines often have employed rotation of the cathode or anode for agitation. Such systems require motorization large enough to carry the final full laden weight of the respective elements and in addition, require direct mechanical connections to be made in a difficult atmosphere. The cathode rotation system also requires that rotation be maintained at a significant speed, most commonly about 170 revolutions per minute (r.p.m.) which may, due to centrifugal forces involved, cause premature flaking from the cathode and may retard plating efficiency at low silver density concentrations. In addition, power transfer to the moving electrode at the high currents necessary is difficult and unreliable.